The screw and nut are basic elements and various kinds of proposals have been presented for them conventionally. The first aspect is regarding screw looseness stoppage. For example, Japanese Utility Model No. SHO58-99513 has proposed a screw in which any one of its female and male threads is formed into a wave-shape. Because when the screw is driven, its wave-shaped protruded portion is deformed elastically so that it presses hard axially, it prevents the screw from being loosened due to a vibration or the like. Further, JP 6-330928 A has proposed a screw in which its threads are constructed with a large pitch and a small pitch alternately for every pitch. When this is fastened with a standard pitch nut, a portion which is pressed strongly by the flank of a thread and deformed is generated thereby preventing the screw from being loosened.
Mr. Hiroshi Fujii, who is one of the inventors of this invention, publicized a step lock bolt as shown in FIG. 19. The flank of a thread of a bolt is provided with fine step portion and slope portion in order to prevent the bolt looseness with plastic deformation of the thread and mating member which occurs with driving of the bolt. These proposals are described in non-patent documents 1 to 3.    Non-patent document 1: Journal of Materials Processing Technology Vol. 56, p 321-332, “Evaluation of Anti-loosening Nuts for Screw Fasteners” H, Fujii et al. 1996    Non-patent document 2: Bulletin of the Japan Society of Mechanical Engineers, series C, volume 62 (597), p 1963-1968, “Development of Screw Fastener Extremely Difficult to Loosen” H, Fujii et al, 1996    Non-patent document 3: Bulletin of the Japan Society of Mechanical Engineers, series C, volume 62 (596), p 1527-1532, “Analysis of Loosening Mechanism of Screw Fastener and Development of Loosening Test Method”, H, Fujii et al, 1996
The second aspect concerns the feed screw mechanism. For example, Japanese Utility Model registered No. 2577786 has disclosed an automobile power seat in which the screw is rotated with a worm decelerator provided motor so as to feed a nut member. The lock when the motor is not driven is achieved by self-lock of the feed screw itself or self-lock of the worm decelerator. Further, JP 5-288253 A has described conventional feed screw mechanisms under three categories easily understandably. A first group converts a rotary motion to a liner motion by sliding rotation between a triangular thread, trapezoidal screw thread or the like and their nut. A second group transmits a drive power through plural steel balls provided in a thread groove like a ball screw. In a third group, with a roller pressed against both flanks of a thread of a screw shaft having a large lead, a member which supports that roller is used like a nut.
However, the above-described conventional screw and nut are premised on that a relative rotation between the both is continuous. A first premise is that as for blocking of looseness between the screw and nut for fastening a certain member, that screw is capable of fastening the member even when its relative rotational position is at any position of 360°, corresponding to a single full turn and further blocking it from being loosened. Although this is preferable from viewpoints of the function, an excessive load is applied to a structure for blocking the looseness. For the reason, the deforming portion of the thread when the screw is tightened does not end with elastic deformation but reaches a plastic deformation depending on the case. This means that if the screw is tightened completely, it cannot be retightened for blocking the looseness. Here, if the way of thinking is turned over, there can be an application field of the screw and nut in which even if the screw cannot be tightened additionally for blocking the looseness when its relative rotational position is located at any position of continuous rotation, its sufficient function is achieved if it can be retightened at a step-like rotation position of every 20° for blocking the looseness, depending on a tightening member or a lead of the screw.
Accordingly, a first object of the present invention is to provide a screw and nut which can be locked only discontinuously but is capable of blocking the looseness securely without applying an excessive stress on a member.
As for the second aspect, the feed screw mechanism, when converting rotary motion to linear motion, the conventional device requests for its continuity and linearity implicitly. The conventional invention intends to search for a higher accuracy or a higher speed on this premise. If converting the way of thinking, the continuity or linearity is not requested depending on application of the feed screw mechanism. For example, in the vehicle power seat, adjustment in a pitch of less than 1 mm is not requested for adjustment of the seat position and seat height. Further, linearity of linear motion to a rotary angle of the drive motor is not requested so much. However, because the employed feed screw mechanism is provided with continuity and linearity, excessive functions are requested for the feed screw mechanism.
For example, the vehicle power seat device disclosed in Japanese Utility Model registered No. 2577786 uses a feed screw device included in a first group mentioned in JP 5-288253 A. In the vehicle power seat device, it is requested to turn off the motor after adjustment of the seat position and maintain that position. Thus, in the vehicle power seat device disclosed in Japanese Utility Model registered No. 2577786, a small lead screw is used as a screw (9) shown in FIG. 1 and it is designed to lock itself with a nut (5). For the reason, the nut (5) is never moved due to a load even when a motor (7) is not powered on. However, because the lead of the screw (9) is small, the screw (9) needs to be rotated at a high speed in order to move the seat comfortably at a high speed, so that a burden applied to the motor (7) and a gear box (8) increases, thereby the price thereof being higher. Even if such an expensive price is permitted, the structure in which the screw (9) and the nut (5) keep a sliding contact with each other produces another inconvenience due to the friction when the screw (9) is rotated at high speeds.
Using the ball screw and roller screw included in a second or third group mentioned in JP 5-288253 A enables solving a problem about the friction between the screw and nut and securing a larger screw lead, thereby achieving a comfortable and quick feed. However, as an opposite effect to the advantage that these screws produce a very small friction, conversion between the rotary motion and the linear motion acts in both ways. That is, the self-lock is disabled. Different from the feed screw device in machine tools using a servo motor as its feed motor, the vehicle power seat device needs some kind of a lock mechanism because the motor power is kept off when it is not used. Thus, not only the device is likely to be expensive, but also if there exists any mechanical mechanism such as a gear between the lock mechanism and the ball screw, there is a fear that comfort of sitting on the seat worsens due to a backlash therebetween or lost motion such as distortion.
Accordingly, a still further object of the present invention is to provide a feed screw device which is capable of achieving a comfortable and quick feed without applying any burden on the motor or gear box and in which the self-lock function is activated when a torque from a driving source is interrupted although not continuously.